For driving an electric compressor having a built-in motor, such a method is often employed that electric current from a DC power source is transformed by an inverter having a semiconductor switching element into pseudo-AC voltage so that the transformed electric current is supplied into the built-in motor of the compressor. The electric current in startup for the electric compressor is often required to be higher than the one at the other case, where the electric current is usually so output from the inverter that the electric compressor is started by the motor electric current which can output the maximum torque required for the startup of the electric compressor (for example, Patent document 1).
The above-described inverter is often affected directly by the temperature of the electric compressor, because it is sometimes incorporated integrally into the electric compressor and sometimes mounted in the neighborhood of the electric compressor. Therefore, when temperature of the electric compressor is high, temperature of the inverter becomes high and temperature of the semiconductor switching element built in the inverter also becomes high. When the semiconductor switching element is at high temperature, its own permitted electric current generally becomes small. Therefore, considering the compressor startup at a high temperature, semiconductor switching whose permitted electric current is rather great should often be used. Use of the semiconductor switching element with a great permitted electric current makes the inverter grow in size and cost, and therefore, makes the built-in inverter type electric compressor grow in size and cost, and even if the inverter is mounted separately with the compressor the compressor as an apparatus including the inverter grows in size or cost.
Patent document 1: JP-A-2006-200507